The invention relates to an image forming apparatus with a fixing device such as a copier, a printer, and a facsimile, for example, to an image forming apparatus that forms by electrophotographic method a toner image(s) on a single side or double sides of a transfer sheet and fixes the image(s) by a fixing device.
As a typical fixing device used for an electrophotographic method of image forming apparatus such as a copier, a printer and a facsimile, a heat roller fixing method has been frequently employed, which comprises a fixing roller with an elastic layer on it which is kept at a preset temperature and a pressing roller with an elastic layer on it which is pressure in contact with the fixing roller and works to heat a transfer sheet having a toner image on it while interposing and conveying the sheet.
Further, there has been a belt-type fixing device that uses an endless belt member (a fixing belt), which is supported by and entrained about a plurality of roller members.
However, when a toner image is heated and fixed by these fixing device, the moisture in the transfer sheet goes away into the air by heat during fixing and consequently, the transfer sheet shrinks. If the sheet shrinks too much, the image size becomes improper and may not meet users' requirements.
Further, due to specific deviations and changes with time of a feeder, the timing to feed sheets to the transfer section moves away from a designed timing value and, consequently, images will be positioned improperly on the sheets.
When forming an image on each side of a transfer sheet for double-side duplication, the transfer sheet subjected to transferring a toner image on one side thereof passes once through a fixing device for fixing the toner image, and then the transfer sheet subjected to transferring another toner image on the other side thereof passes again through the fixing device for fixing the another toner image.
When forming multiple duplication on a single side of transfer sheet to synthesize two images, the transfer sheet subjected to transferring a toner image on one side thereof passes once through a fixing device for fixing the toner image, and then the transfer sheet subjected to transferring another toner image on the same side thereof passes again through the fixing device for fixing the another toner image.
In these double-side duplication and multiple duplication, the first fixed transfer sheet is already shrunk by heat. Consequently, the first and second toner images become different in size and position thereof.
To solve such an image size problem, Patent Document 1 representing Japanese Non-examined Patent Publication H4-288560, for example, discloses a method of to measure the size of a transfer sheet before fixing and the size of the transfer sheet after fixing, to calculate a ratio of shrinkage of the transfer sheet from the result of measurement, and to control the optical system.
When making double-side duplication, the image forming apparatus disclosed by Patent Document 2 representing Japanese Non-examined Patent Publication H10-149057, measures the size of the first transfer sheet before fixing and the size of the transfer sheet after fixing by a sensor, calculates a ratio of shrinkage of the first transfer sheet from the result of measurement, and uses this ratio to correct image sizes for the second and later transfer sheets.
The image forming apparatus disclosed by Patent Document 3 representing Japanese Non-examined Patent Publication H10-319674, has a deviation detecting means that detects a test pattern that is formed on a preset position of a transfer sheet and measures the registration deviation and skew of the transfer sheet and corrects the position of an image to be formed on the other side of the sheet according to the result of the measurement.
The color image forming apparatus disclosed by Patent Document 4 representing Japanese Non-examined Patent Publication H10-319675, is equipped with a first paper size measuring section that measures the size of a transfer sheet before or while an image is formed on one side of the sheet and a second paper size measuring section that measures the size of the transfer sheet before or while an image is formed on the other side of the sheet, and uses the result of these measurements to control the position and length of-the image to be transferred to the transfer sheet.
The image forming apparatus having a Double-Side Copy mode disclosed by Patent Document 5 representing Japanese Non-examined Patent Publication 2002-258680, measures the longitudinal and lateral sizes of a transfer sheet before and after fixing by a measuring means, calculates the ratio of shrinkage of the transfer sheet by the fixing device, and forms a toner image of a magnification corrected by this shrinkage ratio on the photosensitive drum. When making double-side duplication, the image forming apparatus uses the first transfer sheet for calculation of this shrinkage ratio, forms a toner image for one side of the second transfer sheet on the surface of the photosensitive material by a normal image formation method, and forms a toner image for the other side of the second transfer sheet at a magnification equivalent to the shrinkage ratio on the surface of the photosensitive material.
As disclosed by Patent Documents 1 to 5, to correct image location and magnification errors of images due to uneven conveyance of transfer sheets or shrinkage of transfer sheets by fixing, the conventional image forming apparatus has employed a method of forming a test pattern in initial fixing or a method of detecting the leading and trailing edges of a transfer sheet and correcting image location and magnification. The test pattern forming method requires extra transfer sheets and the sheet edge detecting method cannot correct image deviations on the transfer sheets.